1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for imaging a subject utilizing magnetic resonance phenomena.
2. Description of the Related Art
In magnetic resonance imaging (MRI), atomic nucleus spins in biological tissue placed in static magnetic field are excited by a radio wave of Larmor frequency, and an image is reconstructed based on a magnetic resonance signal (MR signal) generated during the excitation. Since a magnetic resonance imaging apparatus (MRI apparatus) for imaging a subject using MRI can provide various types of diagnostic information, such as anatomic diagnostic information, biochemical information and diagnostic function information, it is very important in the field of current diagnostic imaging.
To generate a high-quality image using an MRI apparatus, it is necessary to efficiently detect a weak MR signal output from a subject. To this end, many technical devices are employed. Various types of radio frequency coils (RF coils) for detecting MR signals, such as a saddle type, solenoid type, slot resonator type, and birdcage type, are now available. Further, an optimal type of RF coil is selected and used in accordance with the frequency band, to-be-diagnosed portion, type of magnet, etc. For instance, in a horizontal-magnetic-field MRI apparatus using a cylindrical magnet, a birdcage type coil or saddle type coil is used. In a vertical-magnetic-field MRI apparatus, a solenoid coil is used. For to-be-diagnosed portions of a subject, such as the head and eyeballs, a surface coil excellent in detection sensitivity is used.
MRI apparatuses also require an RF coil for transmitting an RF signal. There is a case where different RF coils are provided for transmission and reception. However, a single RF coil may be used in common for transmission and reception.
For MRI apparatuses, there is a wide-range imaging method for performing imaging while sliding a subject in a direction parallel to the axis of the body. To realize such imaging, a method, for example, has been proposed in which an RF coil is fixed to the top board of a bed using support posts, and a subject placed on a support member is slid between the RF coil and top board (see, for example, U.S. Pat. No. 5,808,468).
FIG. 6 is a perspective view illustrating the subject-moving mechanism described in U.S. Pat. No. 5,808,468.
As shown, a top board 203 is placed on the top surfaces of a bed 201 and auxiliary bed 202. A plurality of rollers 205 are fitted in two grooves 204 formed in the top board 203 in the longitudinal direction of the board. Upper and lower surface coils 206 and 207 are fixed to the top board 203 using support posts 208.
A support plate 209 for placing a subject thereon is provided slidably with respect to the top board 203, supported by the rollers 205. Accordingly, the support plate 209 and a subject (not shown) placed thereon are slidable between the top board 203 and upper surface coil 206. When a worm drive 211 is rotated by a motor 210 mounted on an end of the auxiliary bed 202, the support plate 209 and subject are slid in the longitudinal direction (i.e., in the direction of the body axis of the subject).
By shifting the subject relative to the fixed RF coils 206 and 207, using the top board 203, the portion of the subject positioned near the RF coils 206 and 207, i.e., the portion as a target of imaging, can be changed.
However, during imaging, it is necessary to locate the RF coils 206 and 207 near the center of the static magnetic field generated in the gantry. In the above-described structure, the RF coils 206 and 207 are located with the support plate 209 interposed there between. Since the support plate 209 is thick so as to support the weight of a subject, it is difficult to remove the RF coils 206 and 207. Further, since the RF coil 206 is fixed at the center of the static magnetic field, to move the imaging target portion of the subject to the center of the static magnetic field, the head of the subject may be passed through the center of the static magnetic field. At this time, the head of the subject passes near the RF coil 206, which may well constitute a physiological pressure on the subject.